1. Field of the Invention
The invention relates generally to a compression screw assembly for applying a compressive force to a fractured bone and more specifically to a compression screw system including a lag screw, compression plate and compression screw which can be assembled, aligned and installed so that the lag screw is non-rotatably secured to the compression plate at the option of the surgeon.
2. Description of the Prior Art
Workers in the art have devised various compression screw systems for applying compression to a fractured bone. Generally, the systems include a lag screw which extends from the shaft of the bone through the fracture and is anchored in the head of the bone, a compression plate which is adapted to extend over at least a portion of the head of the lag screw and which is anchored to the shaft of the bone and a compression screw which extends from the compression plate to the lag screw to permit the application of a compressive force between the lag screw and the compression plate.
It is often desirable to assure that the lag screw is non-rotatably secured to the compression plate. Generally this is accomplished in a keyed system by providing the lag screw with a longitudinally directed keyway, and by providing the portion of the hollow barrel of the compression plate that extends over the head of the lag screw with a corresponding longitudinally directed key. The problem with these systems is that it is difficult to insert the compression plate over the lag screw so that the key and keyway are aligned properly because the lag screw is driven completely into the bone before the compression plate is inserted.
One method for alleviating the problem of aligning the lag screw and compression plate is to provide an extension attached to the head of the lag screw to permit the lag screw to be aligned with the plate barrel more easily. A second method is to recess the key of the barrel member away from the front end of the barrel member so that the alignment occurs in two distinct steps: first, the aperture of the barrel member of the compression plate is aligned with the lag screw; and second, the key of the barrel member is aligned with the keyway of the lag screw. In a still further method as shown in U.S. Pat. No. 4,095,591, a barrel guide means having an extension member extending outward of the bone and having a cross section similar to that of the lag screw is used so that the barrel member of the compression plate is aligned on the extension member before it is inserted into the fractured bone.
A major disadvantage of the compression screw systems mentioned above is that the surgeon must determine before the insertion of the lag screw whether to use a keyed system or a non-keyed system. If a keyed system is to be used, the lag screw and compression plate must have the keyway and key, respectively. If a non-keyed system is to be used, the lag screw must be able to freely rotate within the barrel of the compression plate.
Accordingly, there exists a need for a compression screw assembly wherein the parts are properly aligned and installed and thereafter the lag screw may be non-rotatably secured to the compression plate at the option of the surgeon.